Structures of all types are susceptible to fatigue cracks that generally initiate at fastener holes. The causes of initiation are many and varied. Early detection of fatigue cracks and corrective action combine to mitigate the risks and the expense of maintaining structures. Present inspection methods for fatigue cracks generally depend on either the crack being sufficiently large for its detection by visual or radiographic methods, or they require the removal of the fastener to locate smaller cracks or flaws using eddy currents. The cost of hole inspection includes not only the time of removal and replacement, but also a factor for possible damage incurred solely from the act of removal.
It is known to use ultrasonic means to inspect for flaws in bodies. In U.S. Pat. No. 3,280,621 there is a search mechanism for detecting flaws in metal where several transducers are arranged in a holding head capable of being spun. The spinning head holding the transducers moves over the material to be checked while both head and material to be tested are immersed in a fluid. U.S. Pat. No. 3,431,774 also tests, in this case, sheet material immersed in a liquid medium with a moving rotating head adapted to generate a plurality of ultrasonic beams while immersed in the liquid. These patents are typical of ultrasonic testing devices in that a transducer and material to be tested are both immersed in a fluid and the transducer rotates while moving over the material to be tested, and thus inspects a large area of material.